Ultrasound Physics & Instrumentation
4th Edition
Volume I
Companion Presentation
Frank R. Miele
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License Agreement
This presentation is the sole property of
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No part of this presentation may be copied or used for any purpose other than
as part of the partnership program as described in the license agreement.
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All Copyright Laws Apply.
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Volume I Outline
 Chapter 1: Mathematics
 Chapter 2: Waves
 Chapter 3: Attenuation
 Chapter 4: Pulsed Wave
 Chapter 5: Transducers
 Chapter 6: System Operation
 Level 1
 Level 2
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Chapter 6: System Operation - Level 2
In Level 1 we discussed general signal processing including the 5 receiver
functions, the function of receiver gain, the basics of TGC, and the concept of
SNR.
Level 2 focuses on:
 overall system design
 a more in depth treatment of compensation
 scan conversion
 the function of compression
 measurements
 video display
 data storage
 more complex approaches to image production
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System Block Diagram
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Fig. 22: (Pg 323)
Effects of Varying Focal Depth
In Figure 23, the focus is set to shallow (at a depth of 4 cm). In Figure 24, the
focus is set appropriately at 12 cm. In Figure 25, the focus is set too deep, at a
depth of 17 cm. Notice the difference in the near field and the far field with the
varying focus.
Fig 23
Fig 24
(Pg 325)
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Fig 25
Focal Depth (from Animation CD)
(Pg 325 B)
(Pg 325 A)
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Internal TGC
Some TGC is set internal to the ultrasound system, reducing the range of TGC
necessary to be controlled by the user. If an internal TGC profile were not used, the
TGC controls would be hypersensitive; even small changes in TGC slider positions
would result in large changes in gain as shown in Figure 27.
(Pg 326)
Fig. 27: Without internal TGC
Fig. 26 : Normal TGC range
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Pre-compensated TGC Profiles
Most systems now allow the user to choose pre-compensated TGC profiles. In this
mode, a default TGC profile is applied, based on imaging parameters such as depth,
frequency, and preset. If the default setting is correct, the user would set the
external TGC in the center as shown in Figure 28. In general, small adjustments are
required to correctly adjust the default profile, as shown in Figure 29.
(Pg 327)
Fig. 29: Without internal TGC
Fig. 28 : Normal TGC range
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Effects of Setting Overall Gain Too High
Fig. 30: (Pg 328)
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Effects of Setting Overall Gain Too High
(from Animation CD)
(Pg 328)
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Effects of Setting Overall Gain Too Low
Fig. 31: (Pg 329)
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Effects of Setting Overall Gain Too Low
(from Animation CD)
(Pg 329 A)
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Results of Incorrect TGC Settings
(from Animation CD)
(Pg 329 B)
Note that a single TGC slider affects the zone above and below, as well as the
actual depth zone it represents since the gain profiled is ”smoothed” over
depth.
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Lower and Higher Frequency Profiles
Fig. 32:
Fig. 33:
(Pg 330)
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These two TGC profiles
represent the difference
that occurs using a
lower (Figure 32) vs.
higher (Figure 33)
frequency transducer.
The assumption must
be that the imaging is
taking place on the
same patient, and
imaging to the same
depth. Since higher
frequency attenuates
faster, the TGC profile
is steeper.
Creating B-mode from A-mode
(Example 1)
Fig. 34a: (Pg 333)
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Creating B-mode from A-mode
(Example 2)
Fig. 34b: (Pg 334)
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Creating B-mode from A-mode
(Example 3)
Fig. 34c: (Pg 334)
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Preprocessing and Post Processing
Revisited
Preprocessing:
Takes place before scan conversion and cannot be changed on data that
has been stored in the internal system memory (cine memory) (for
conventional imaging systems).
Post Processing:
Takes place after scan conversion and can be changed after the data has
been stored in the internal system memory (cine memory).
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Grayscale and Visual Dynamic Range
The dynamic range of the human eye is less than 36 dB, which is
equivalent to 64 shades of gray simultaneously.
16 Shades
32 Shades
64 Shades
Fig. 36: (Pg 338)
Since the dynamic range of signals returning from the body are so much
greater than the dynamic range of the human eye, non-linear (logarithmic)
compression must be used.
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Effects of Compression
This figure demonstrates both the concept of compression and a potential issue
with compression. Notice how the signals from the mass and tissue are much
closer together after compression, possibly resulting in an inability to distinguish
the mass from the tissue.
Fig. 37: (Pg 339)
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Compression (Animation)
(Pg 339)
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Effect of Compression Maps
The next six slides show the same cardiac image and the same vascular
image using different compression (grayscale) maps.
The actual grayscale map used to create the images is shown between the
cardiac and vascular image. The input signal is on the horizontal axis (x-axis)
and the output signal (the signal brightness) is plotted on the vertical axis (yaxis).
Interesting Note:
If you compare the images from the next 6 slides with the images as presented
in the book, you will notice that there is significantly less contrast apparent in
the book images than the PowerPoint images, even though the images are
identical. This shows how much the display medium can affect the appearance
of grayscaled data.
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Map 1
Of the six maps displayed, this map uses the most dynamic range for
higher-level signals. Mid-level and weaker signals are all mapped to
very dark shades and hence are not very visible. As a result, only the
strongest signals are visible in these images, making the images appear
relatively dark.
(Pg 340)
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Map 2
Of the six maps displayed, this map uses the most dynamic range for the
mid-range signals. Again notice that very weak signals are not very
visible since low-level signals are mapped to very dark values.
However, since the mid-range signals are mapped to lighter shades, the
image has a much “brighter” appearance than map 1.
(Pg 340)
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Map 3
In comparison to the previous map, the transition begins earlier so that
weaker signals are now mapped into a visible range. Since more of the
dynamic range is used for lower level signals, notice that the stronger
signals are being mapped to a much brighter range, yielding less
distinction between the stronger signals in the images.
(Pg 341)
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Map 4
Of the six maps, this map most dramatically eliminated low-level signals.
This map effectively acts like a “reject”, mapping low-level signals such
as noise to black. However, the results of such a dramatic map is that
any weak signals, such as occur with thrombus, will almost never
become visible.
(Pg 341)
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Map 5
This map rises rapidly for low level signals, and uses less dynamic range
for mid-level signals. Notice how the low level signals are now visible.
For the first time, a thrombus is clearly visible in the popliteal vein.
Although this maps results in less distinction between tissue and
stronger specular reflectors, the benefit of using a map like this is clear if
you consider that the first four maps used would have resulted in missing
the thrombus.
(Pg 341)
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Map 6
This last map has the most dramatic rise in output intensity for very lowlevel signals. Since so much of the dynamic range is used for weak
signals, there is very little DNR left for the mid-range and higher-level
signals. Notice how obvious the thrombus now becomes.
(Pg 341)
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Effects of Compression (Animation)
(Pg 339)
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Effects of Compression Settings
This image again shows the importance of periodically changing the
compression maps when scanning so as to minimize the risk of missing
lower level signals that can occur (such as from thrombus and masses).
In the right image, the thrombus is appreciated in the right popliteal
whereas in the left image, the thrombus is not seen.
Fig. 38: (Pg 342)
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Effects of Compression Settings
This image is the same popliteal vein as in the previous slide but shown
in transverse view. Again, the distinction between visualization and the
lack of visualization comes from changing the compression setting on
the ultrasound system.
Fig. 39: (Pg 343)
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Compression (Animation)
(Pg 343 A)
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Tissue Colorization
As discussed earlier, compression is required since the human eye is
only capable of seeing fewer than 64 shades of gray simultaneously.
Since the eye is capable of seeing more levels of color, colorization is
sometimes used to extend the visual dynamic range. Colorization does
not always result in better visualization of signals, but does periodically
provide some advantage.
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Tissue Colorization (Animation)
(Pg 343 B)
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Tissue Colorization: Case 2
(Pg 343 C)
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Tracing an Area
When tracing a structure, angle must be considered so as to not over or
underestimate the true area.
Fig. 40a: Area Overestimation
Fig. 40b: Area Over and Underestimation
(Pg 344)
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Area Measurement Error (Animation)
(Pg 345)
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Longitudinal Plane and Area
Underestimation
Fig. 42: (Pg 345)
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Non-linear Area Errors
When measuring a radius for an area calculation, extreme caution must
be exercised since an error in the radius measurement is squared when
calculating the area.
Fig. 43: (Pg 346)
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Video Display and Monitors
The performance characteristics of the display monitor can affect the
perceived data in ultrasound. As a result, it is important to discuss
monitor formats. In recent years, the format in the U.S. has been
changing from an interlaced to non-interlaced format.
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Interlaced Monitors
Although non-interlaced monitors are much more common now,
interlaced monitors have been the standard for many years. An
interlaced monitor presents an image as the combination of two
interlaced fields, an odd field and an even field.
Fig. 44 & 45: (Pg 347)
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Interlaced Monitors and Frame Rate
Since each field of an interlaced monitor requires 1/60th of a second to
display, an entire frame requires 1/30th of a second to display. To convert
the frame time of 1/30th of a second into a frame rate, we take the
reciprocal, yielding 30 Hz. In other words, an interlaced monitor can
display only 30 frames per second.
F ram e t im e :
1
sec

60 odd field
1
sec

60 even field
2
sec
60 fra m e
F ram e rate :
1
fram e tim e

30 fram e
 30 H z
se c
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
1
sec
3 0 fram e
Non-interlaced Monitors
A non-interlaced monitor is able to display both fields on an image
simultaneously. As a result, the image is produced in half of the time,
and the frame rate is double that of an interlaced monitor. So a noninterlaced monitor is capable of displaying 60 frames per second.
Fig. 46: (Pg 348)
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Pixel Size and Resolution
Larger Pixels (Worse Resolution)
Fig. 47: (Pg 349)
Smaller Pixels (Better Resolution)
Fig. 48: (Pg 349)
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Pixel Brightness (Bit Levels)
Fig. 49: (Pg 350)
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Relating Brightness Levels to Binary
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Fig. 50: (Pg 350)
Brightness Levels and Ambient Light
Animation
(Pg 351)
The affects of ambient light are important since low level signals can be missed
in brighter light, and signals stored to storage devices might not match what is
visualized on the monitor.
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Zoom (Res Mode, Magnification)
“Zoom” is a technique designed to accommodate the desire to visualize
regions of images in a larger format. There are two common
approaches to achieve a “zoomed” image: acoustic zoom (“write”
zoom) and non-acoustic (“read” zoom).
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Non-acoustic (Read) Zoom
Fig. 51a and 51b: (Pg 355)
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Non-Acoustic Zoom
Fig. 52a and 52b: (Pg 355)
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Acoustic Zoom (“write zoom”)
Unlike a non-acoustic zoom, an acoustic zoom re-transmits acoustic
lines, potentially using different line density, to achieve better resolution.
Fig. 53: (Pg 356)
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Acoustic Zoom
Fig. 54a: (Pg 356)
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Acoustic vs. Non-Acoustic Zoom
Non-Acoustic
Acoustic
Fig. 55a and 55b: (Pg 357)
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Alternatives to Conventional B-Mode Imaging
There are many alternative approaches to transmitting a single acoustic
line per each individual display line. We will discuss 2 of these
techniques:
 multiple transmit foci
 parallel processing
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Multiple Transmit Foci
Using multiple transmit foci improves lateral resolution. Since each display line
is comprised of multiple acoustic lines, using multiple foci, the frame rate is
often significantly slower, decreasing temporal resolution.
Fig. 56: (Pg 358)
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Temporal Resolution and Multiple Foci
Notice that although
there are 3 transmit
foci in this example,
the frame time is not
a full three times
longer than for a
single focus. This is
because three foci
does not mean three
full lines but rather
one short line, one
intermediate line,
and one full line as
shown in this figure.
Fig. 57: (Pg 359)
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Multiple Transmit Foci (Animation)
(Pg 358)
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Banding Noise
Fig. 58: (Pg 360)
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Dynamic Receive Focusing
Since transmitting is active, there can only be one focus per transmitted line.
Unlike transmitting, receiving is “passive”, so there can be continuous, dynamic
receive focusing. Better lateral resolution can be achieved by dynamically
changing the focus on receive, instead of having one fixed receive focal depth.
Fig. 59: (Pg 360)
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Continuous Variable (Dynamic) Receive
Focusing
Fig. 60: (Pg 361)
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Parallel Processing
Parallel processing is a
technique of transmitting less
focused “fatter” beams and
then receiving multiple
simultaneous “narrower”
beams. In the simplest
approach, one wider beam is
transmitted and two beams are
received. This technique can
be used to improve frame rate
(temporal resolution) or to
increase line density.
Fig. 61: (Pg 362)
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Averaging Based Techniques
There are many averaging based techniques used in ultrasound. Averaging
can be advantageous since, averaging increases the signal level faster relative
to the increase in the noise level (improved signal to noise ratio (SNR)).
Im provem ent in S N R =
n
n
Fig. 62: (Pg 363)
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
n
Improved SNR from Signal Coherence
Notice that in the frame on the left, there are two signals of identical amplitude
that align perfectly (constructive interference). Also notice that the noise from the
two signals is neither perfectly in phase or perfectly out of phase (partial
constructive interference). Therefore, the signals add to create one signal twice
as large, whereas the noise only partially adds to create noise 1.4 times as large
(the square root of 2).
Fig. 63 and 64: (Pg 364)
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Averaging and SNR
Noisy Image
16 Images Averaged
2 Images Averaged
9 Images Averaged
Image Without Noise
Fig. 65: (Pg 365)
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Averaging and SNR Case 1 (Animation)
(Pg 365 A)
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Averaging and SNR Case 2 (Animation)
(Pg 365 B)
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Averaging and SNR Case 3 (Animation)
(Pg 365 C)
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Compounding Imaging
Compound imaging is produced by averaging multiple frames, each
transmitted at varying angles. The multiple frames improve the SNR,
and the varying angles diminish the number of specular based image
artifacts that appear in the image.
Fig. 66: (Pg 366)
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Compound Imaging Example
Compound Image
Conventional Image
Fig. 67: (Pg 367)
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Compound Imaging Case 1 (Animation)
(Pg 367 A)
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Compound Imaging Case 2 (Animation)
(Pg 367 B)
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Spatial Averaging Techniques
Spatial Averaging
reduces noise by
“adjusting” pixels
according to an
algorithm that looks
at the values of the
nearest neighbors,
as shown in this
example. The
resultant “smoothed”
image is shown on
the next slide.
Fig. 68: (Pg 368)
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Spatially Smoothed Image
By comparison with the image of
the previous slide, notice this image
is more uniform. In essence, the
localized average behavior is used
to reduce the variation from pixel to
pixel.
Fig. 69: (Pg 369)
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3-Dimensional imaging
3-D imaging is now available on many different ultrasound machines.
One of the greatest challenges to overcome with 3-D is creating the
means by which to display all three dimensions of data. Many believe
that eventually 3-D will replace 2D, just as 2D has replaced A-mode.
Fig. 70: (Pg 370)
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M-Mode
M-Mode is generally used in cardiac imaging and for fetal heart rates. Mmode (motion mode) is a non-scanned modality. By transmitting repeatedly
in the same direction and displaying the grayscaled values of each line over
time, motion can be discerned as with this mitral valve tracing.
Fig. 71: (Pg 371)
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Review of Resolution
Fig. 72: (Pg 371)
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NOTES:
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